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# Find a peak element in a 2D array

• Difficulty Level : Medium
• Last Updated : 23 Feb, 2023

An element is a peak element if it is greater than or equal to its four neighbors, left, right, top and bottom. For example neighbors for A[i][j] are A[i-1][j], A[i+1][j], A[i][j-1] and A[i][j+1]. For corner elements, missing neighbors are considered of negative infinite value. The task is to find the index of the peak element.

Examples:

Input: 10 20 15
21 30 14
7  16 32
Output: 1, 1
Explanation: The value at index {1, 1} is 30, which is a peak element because all its neighbors are smaller or equal to it. Similarly, {2, 2} can also be picked as a peak.

Input: 10 7
11 17
Output : 1, 1

1.  A Diagonal adjacent is not considered a neighbour.
2. A peak element is not necessarily the maximal element.
3. More than one such element can exist.
4. There is always a peak element. We can see this property by creating some matrices using pen and paper.

Method 1: (Brute Force) :

Iterate through all the elements of Matrix and check if it is greater/equal to all its neighbours. If yes, return the element.

## C++

 `// Finding peak element in a 2D Array.` `#include ` `using` `namespace` `std;`   `vector<``int``> findPeakGrid(vector> arr){` `    ``vector<``int``> result;` `    ``int` `row = arr.size();` `    ``int` `column = arr[0].size();` `    `  `    ``for``(``int` `i = 0; i 0)` `                ``if``(arr[i][j] < arr[i-1][j]) ``continue``;` `            ``// checking with right element` `            ``if``(j < column-1)` `                ``if``(arr[i][j] < arr[i][j+1]) ``continue``;` `            ``// checking with bottom element` `            ``if``(i < row-1)` `                ``if``(arr[i][j] < arr[i+1][j]) ``continue``;` `            ``// checking with left element` `            ``if``(j > 0)` `                ``if``(arr[i][j] < arr[i][j-1])  ``continue``;` `            `  `            ``result.push_back(i);` `            ``result.push_back(j);` `            ``break``;` `        ``}` `    ``}` `    ``return` `result;` `}`   `// Driver Code` `int` `main(){` `    ``vector> arr = {{9,8}, {2,6}};` `    ``vector<``int``> result = findPeakGrid(arr);` `    ``cout<<``"Peak element found at index: "``<

## Java

 `import` `java.util.*;`   `public` `class` `Main {` `    ``public` `static` `List findPeakGrid(``int``[][] arr)` `    ``{` `        ``List result = ``new` `ArrayList<>();` `        ``int` `row = arr.length;` `        ``int` `column = arr[``0``].length;`   `        ``for` `(``int` `i = ``0``; i < row; i++) {` `            ``for` `(``int` `j = ``0``; j < column; j++) {` `                ``// checking with top element` `                ``if` `(i > ``0``)` `                    ``if` `(arr[i][j] < arr[i - ``1``][j])` `                        ``continue``;` `                ``// checking with right element` `                ``if` `(j < column - ``1``)` `                    ``if` `(arr[i][j] < arr[i][j + ``1``])` `                        ``continue``;` `                ``// checking with bottom element` `                ``if` `(i < row - ``1``)` `                    ``if` `(arr[i][j] < arr[i + ``1``][j])` `                        ``continue``;` `                ``// checking with left element` `                ``if` `(j > ``0``)` `                    ``if` `(arr[i][j] < arr[i][j - ``1``])` `                        ``continue``;`   `                ``result.add(i);` `                ``result.add(j);` `                ``break``;` `            ``}` `        ``}` `        ``return` `result;` `    ``}`   `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``int``[][] arr = { { ``9``, ``8` `}, { ``2``, ``6` `} };` `        ``List result = findPeakGrid(arr);` `        ``System.out.println(``"Peak element found at index: "` `                           ``+ result.get(``0``) + ``", "` `                           ``+ result.get(``1``));` `    ``}` `}`

## Python3

 `# Finding a peak element in 2D array` `def` `findPeakGrid(arr):` `    ``result ``=` `[]` `    ``row ``=` `len``(arr)` `    ``column ``=` `len``(arr[``0``])` `    `  `    ``for` `i ``in` `range``(row):` `        ``for` `j ``in` `range``(column):` `            `  `            ``# checking with top element` `            ``if` `i > ``0``:` `                ``if` `arr[i][j] < arr[i``-``1``][j]:` `                    ``continue` `            ``# checking with right element` `            ``if` `j < column``-``1``:` `                ``if` `arr[i][j] < arr[i][j``+``1``]:` `                    ``continue` `            ``# checking with bottom element` `            ``if` `i < row``-``1``:` `                ``if` `arr[i][j] < arr[i``+``1``][j]:` `                    ``continue` `            ``# checking with left element` `            ``if` `j > ``0``:` `                ``if` `arr[i][j] < arr[i][j``-``1``]:` `                    ``continue` `            `  `            ``result.append(i)` `            ``result.append(j)` `            ``break`   `    ``return` `result`   `# driver code` `arr ``=` `[[``9``,``8``], [``2``,``6``]]` `result ``=` `findPeakGrid(arr)` `print``(``"Peak element found at index:"``, result)`   `# This code is constributed by phasing17`

## C#

 `// C# code to find peak element in a 2D array` `using` `System;` `using` `System.Collections.Generic;` `class` `GFG {` `  ``static` `int``[] findPeakGrid(``int``[][] arr){` `    ``int``[] result = ``new` `int``[2];` `    ``int` `row = arr.Length;` `    ``int` `column = arr[0].Length;` `    ``for``(``int` `i = 0; i 0)` `          ``if``(arr[i][j] < arr[i-1][j]) ``continue``;`   `        ``// checking with right element` `        ``if``(j < column-1)` `          ``if``(arr[i][j] < arr[i][j+1]) ``continue``;`   `        ``// checking with bottom element` `        ``if``(i < row-1)` `          ``if``(arr[i][j] < arr[i+1][j]) ``continue``;`   `        ``// checking with left element` `        ``if``(j > 0)` `          ``if``(arr[i][j] < arr[i][j-1])  ``continue``;` `        ``result[0] = i;` `        ``result[1] = j;` `        ``break``;` `      ``}` `    ``}` `    ``return` `result;` `  ``}`   `  ``// driver code to test above function` `  ``public` `static` `void` `Main(){` `    ``int``[][] arr = { ``new``[] { 9, 8 }, ``new``[] { 2, 6 } };` `    ``int``[] result = findPeakGrid(arr);` `    ``Console.WriteLine(``"Peak element found at index: "` `                      ``+ result[0] + ``","` `+ result[1]);` `  ``}` `}`   `// THIS CODE IS CONTRIBUTED BY YASH AGARWAL(YASHAGAWRAL2852002)`

## Javascript

 `// Finding a peak element in 2D array` `function` `findPeakGrid(arr){` `    ``let result = [];` `    ``let row = arr.length;` `    ``let column = arr[0].length;` `    `  `    ``for``(let i = 0; i 0)` `                ``if``(arr[i][j] < arr[i-1][j]) ``continue``;` `            ``// checking with right element` `            ``if``(j < column-1)` `                ``if``(arr[i][j] < arr[i][j+1]) ``continue``;` `            ``// checking with bottom element` `            ``if``(i < row-1)` `                ``if``(arr[i][j] < arr[i+1][j]) ``continue``;` `            ``// checking with left element` `            ``if``(j > 0)` `                ``if``(arr[i][j] < arr[i][j-1])  ``continue``;` `            `  `            ``result.push(i);` `            ``result.push(j);` `            ``break``;` `        ``}` `    ``}` `    ``return` `result;` `}`   `// driver code` `let arr = [[9,8], [2,6]];` `let result = findPeakGrid(arr);` `console.log(``"Peak element found at index: "` `+ result[0] + ``", "` `+ result[1]);` `// THIS CODE IS CONTRIBUTED BY KIRTI AGARWAL(KIRTIAGARWAL23121999)`

Output

```Peak element found at index: 0, 0
```

Time Complexity: O(rows * columns)
Auxiliary Space: O(1)

Method 2 : (Efficient):

This problem is mainly an extension of Find a peak element in 1D array. We apply similar Binary Search based solution here.

1. Consider mid column and find maximum element in it.
2. Let index of mid column be ‘mid’, value of maximum element in mid column be ‘max’ and maximum element be at ‘mat[max_index][mid]’.
3. If max >= A[index][mid-1] & max >= A[index][mid+1], max is a peak, return max.
4. If max < mat[max_index][mid-1], recur for left half of matrix.
5. If max < mat[max_index][mid+1], recur for right half of matrix.

Below is the implementation of the above algorithm:

## C++

 `// Finding peak element in a 2D Array.` `#include ` `using` `namespace` `std;`   `// Finding peak element in a 2D Array.` `#include ` `using` `namespace` `std;`   `vector<``int``> findPeakGrid(vector >& mat)` `{`   `    ``int` `stcol = 0,` `        ``endcol` `        ``= mat[0].size()` `          ``- 1; ``// Starting point & end point of Search Space`   `    ``while` `(stcol <= endcol) { ``// Bin Search Condition`   `        ``int` `midcol = stcol + (endcol - stcol) / 2,` `            ``ansrow = 0;` `        ``// "ansrow" To keep the row number of global Peak` `        ``// element of a column`   `        ``// Finding the row number of Global Peak element in` `        ``// Mid Column.` `        ``for` `(``int` `r = 0; r < mat.size(); r++) {` `            ``ansrow = mat[r][midcol] >= mat[ansrow][midcol]` `                         ``? r` `                         ``: ansrow;` `        ``}`   `        ``// Finding next Search space will be left or right` `        ``bool` `valid_left = midcol - 1 >= stcol` `                          ``&& mat[ansrow][midcol - 1]` `                                 ``> mat[ansrow][midcol];` `        ``bool` `valid_right = midcol + 1 <= endcol` `                           ``&& mat[ansrow][midcol + 1]` `                                  ``> mat[ansrow][midcol];`   `        ``// if we're at Peak Element` `        ``if` `(!valid_left && !valid_right) {` `            ``return` `{ ansrow, midcol };` `        ``}`   `        ``else` `if` `(valid_right)` `            ``stcol = midcol` `                    ``+ 1; ``// move the search space in right` `        ``else` `            ``endcol = midcol` `                     ``- 1; ``// move the search space in left` `    ``}`   `    ``return` `{ -1, -1 };` `}`     `// Driver Code` `int` `main()` `{` `    ``vector > arr = {{9, 8}, {2 ,6}};`   `    ``vector<``int``> result = findPeakGrid(arr);`   `    ``cout << ``"Peak element found at index: "` `<< result[0] << ``","` `<< result[1] << endl;` `    ``return` `0;` `}`

## Java

 `// Finding peak element in a 2D Array.` `import` `java.util.*;`   `public` `class` `GFG {`   `  ``static` `int``[] findPeakGrid(``int``[][] mat)` `  ``{`   `    ``// Starting point & end point of Search Space` `    ``int` `stcol = ``0``, endcol = mat[``0``].length - ``1``;`   `    ``// Bin Search Condition` `    ``while` `(stcol <= endcol) {`   `      ``int` `midcol = stcol + (endcol - stcol) / ``2``,` `      ``ansrow = ``0``;` `      ``// "ansrow" To keep the row number of global` `      ``// Peak element of a column`   `      ``// Finding the row number of Global Peak element` `      ``// in Mid Column.` `      ``for` `(``int` `r = ``0``; r < mat.length; r++) {` `        ``ansrow` `          ``= mat[r][midcol] >= mat[ansrow][midcol]` `          ``? r` `          ``: ansrow;` `      ``}`   `      ``// Finding next Search space will be left or` `      ``// right` `      ``boolean` `valid_left` `        ``= midcol - ``1` `>= stcol` `        ``&& mat[ansrow][midcol - ``1``]` `        ``> mat[ansrow][midcol];` `      ``boolean` `valid_right` `        ``= midcol + ``1` `<= endcol` `        ``&& mat[ansrow][midcol + ``1``]` `        ``> mat[ansrow][midcol];`   `      ``// if we're at Peak Element` `      ``if` `(!valid_left && !valid_right) {` `        ``return` `new` `int``[] { ansrow, midcol };` `      ``}`   `      ``else` `if` `(valid_right)` `        ``stcol` `        ``= midcol` `        ``+ ``1``; ``// move the search space in right` `      ``else` `        ``endcol` `        ``= midcol` `        ``- ``1``; ``// move the search space in left` `    ``}`   `    ``return` `new` `int``[] { -``1``, -``1` `};` `  ``}`   `  ``// Driver Code` `  ``public` `static` `void` `main(String[] args)` `  ``{` `    ``int``[][] arr = { { ``9``, ``8` `}, { ``2``, ``6` `} };`   `    ``int``[] result = findPeakGrid(arr);`   `    ``System.out.println(``"Peak element found at index: "` `                       ``+ result[``0``] + ``","` `+ result[``1``]);` `  ``}` `}`   `// This code is contributed by Karandeep1234`

## Python3

 `# Finding peak element in a 2D Array.` `def` `findPeakGrid(mat):` `    ``stcol ``=` `0` `    ``endcol ``=` `len``(mat[``0``]) ``-` `1``; ``# Starting po  end po of Search Space`   `    ``while` `(stcol <``=` `endcol):  ``# Bin Search Condition`   `        ``midcol ``=` `stcol ``+` `int``((endcol ``-` `stcol) ``/` `2``)` `        ``ansrow ``=` `0``;` `        ``# "ansrow" To keep the row number of global Peak` `        ``# element of a column`   `        ``# Finding the row number of Global Peak element in` `        ``# Mid Column.` `        ``for` `r ``in` `range``(``len``(mat)):` `            ``ansrow ``=` `r ``if` `mat[r][midcol] >``=` `mat[ansrow][midcol] ``else` `ansrow;` `        `    `        ``# Finding next Search space will be left or right` `        ``valid_left ``=`  `midcol ``-` `1` `>``=` `stcol ``and` `mat[ansrow][midcol ``-` `1``] > mat[ansrow][midcol];` `        ``valid_right ``=` `midcol ``+` `1` `<``=` `endcol ``and` `mat[ansrow][midcol ``+` `1``] > mat[ansrow][midcol];`   `        ``# if we're at Peak Element` `        ``if` `(``not` `valid_left ``and` `not` `valid_right) :` `            ``return` `[ ansrow, midcol ];` `        `    `        ``elif` `(valid_right):` `            ``stcol ``=` `midcol  ``+` `1``; ``# move the search space in right` `        ``else``:` `            ``endcol ``=` `midcol  ``-` `1``; ``# move the search space in left` `    `  `    ``return` `[ ``-``1``, ``-``1` `];`   `# Driver Code` `arr ``=` `[[``9``, ``8``], [``2` `,``6``]];` `result ``=` `findPeakGrid(arr);` `print``(``"Peak element found at index:"``, result)`   `# This code is contributed by phasing17.`

## C#

 `// Finding peak element in a 2D Array.` `using` `System;` `using` `System.Collections.Generic;`   `public` `class` `GFG {`   `  ``static` `int``[] findPeakGrid(``int``[][] mat)` `  ``{`   `    ``// Starting point & end point of Search Space` `    ``int` `stcol = 0, endcol = mat[0].Length - 1;`   `    ``// Bin Search Condition` `    ``while` `(stcol <= endcol) {`   `      ``int` `midcol = stcol + (endcol - stcol) / 2,` `      ``ansrow = 0;` `      ``// "ansrow" To keep the row number of global` `      ``// Peak element of a column`   `      ``// Finding the row number of Global Peak element` `      ``// in Mid Column.` `      ``for` `(``int` `r = 0; r < mat.Length; r++) {` `        ``ansrow` `          ``= mat[r][midcol] >= mat[ansrow][midcol]` `          ``? r` `          ``: ansrow;` `      ``}`   `      ``// Finding next Search space will be left or` `      ``// right` `      ``bool` `valid_left = midcol - 1 >= stcol` `        ``&& mat[ansrow][midcol - 1]` `        ``> mat[ansrow][midcol];` `      ``bool` `valid_right = midcol + 1 <= endcol` `        ``&& mat[ansrow][midcol + 1]` `        ``> mat[ansrow][midcol];`   `      ``// if we're at Peak Element` `      ``if` `(!valid_left && !valid_right) {` `        ``return` `new` `int``[] { ansrow, midcol };` `      ``}`   `      ``else` `if` `(valid_right)` `        ``stcol` `        ``= midcol` `        ``+ 1; ``// move the search space in right` `      ``else` `        ``endcol` `        ``= midcol` `        ``- 1; ``// move the search space in left` `    ``}`   `    ``return` `new` `int``[] { -1, -1 };` `  ``}`   `  ``// Driver Code` `  ``public` `static` `void` `Main(``string``[] args)` `  ``{` `    ``int``[][] arr = { ``new``[] { 9, 8 }, ``new``[] { 2, 6 } };`   `    ``int``[] result = findPeakGrid(arr);`   `    ``Console.WriteLine(``"Peak element found at index: "` `                      ``+ result[0] + ``","` `+ result[1]);` `  ``}` `}`   `// This code is contributed by phasing17`

## Javascript

 `// Finding peak element in a 2D Array.`   `function` `findPeakGrid(mat)` `{`   `    ``let stcol = 0,  endcol = mat[0].length - 1; ``// Starting po  end po of Search Space`   `    ``while` `(stcol <= endcol) { ``// Bin Search Condition`   `        ``let midcol = stcol + Math.floor((endcol - stcol) / 2), ansrow = 0;` `        ``// "ansrow" To keep the row number of global Peak` `        ``// element of a column`   `        ``// Finding the row number of Global Peak element in` `        ``// Mid Column.` `        ``for` `(let r = 0; r < mat.length; r++) {` `            ``ansrow = mat[r][midcol] >= mat[ansrow][midcol] ? r : ansrow;` `        ``}`   `        ``// Finding next Search space will be left or right` `        ``let valid_left =  midcol - 1 >= stcol && mat[ansrow][midcol - 1] > mat[ansrow][midcol];` `        ``let valid_right = midcol + 1 <= endcol && mat[ansrow][midcol + 1] > mat[ansrow][midcol];`   `        ``// if we're at Peak Element` `        ``if` `(!valid_left && !valid_right) {` `            ``return` `[ ansrow, midcol ];` `        ``}`   `        ``else` `if` `(valid_right)` `            ``stcol = midcol  + 1; ``// move the search space in right` `        ``else` `            ``endcol = midcol  - 1; ``// move the search space in left` `    ``}`   `    ``return` `[ -1, -1 ];` `}`     `// Driver Code` `let arr = [[9, 8], [2 ,6]];`   `let result = findPeakGrid(arr);`   `console.log(``"Peak element found at index: "` `+ result[0] + ``","` `+ result[1])`   `// This code is contributed by phasing14.`

Output

`Peak element found at index: 0,0`

Time Complexity : O(rows * log(columns)). We recur for half the number of columns. In every recursive call, we linearly search for the maximum in the current mid column.
Auxiliary Space: O(columns/2) for Recursion Call Stack

This article is contributed by Rohit Thapliyal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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